Long term evolution (LTE) based on 3rd generation partnership project (3GPP) technical specification (TS) release 8 is a promising next-generation mobile communication standard.
As disclosed in 3GPP TS 36.211 V8.7.0 (2009-05) “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, a physical channel of the LTE can be classified into a downlink channel, i.e., a physical downlink shared channel (PDSCH) and a physical downlink control channel (PDCCH), and an uplink channel, i.e., a physical uplink shared channel (PUSCH) and a physical uplink control channel (PUCCH).
The PUCCH is an uplink control channel used for transmission of an uplink control signal such as a hybrid automatic repeat request (HARQ) positive-acknowledgement (ACK)/negative-acknowledgement (NACK) signal, a channel quality indicator (CQI), and a scheduling request (SR).
Meanwhile, 3GPP LTE-advanced (A) which is an evolution of 3GPP LTE is under development. Examples of techniques employed in the 3GPP LTE-A include carrier aggregation and multiple input multiple output (MIMO) supporting four or more antenna ports.
The carrier aggregation uses a plurality of component carriers. The component carrier is defined with a center frequency and a bandwidth. One uplink component carrier and one downlink component carrier are mapped to one cell. When a user equipment receives a service by using a plurality of downlink component carriers, it can be said that the user equipment receives the service from a plurality of serving cells.
With the introduction of the carrier aggregation and the MIMO, it is required to increase capacity of a control channel. The increase in the number of downlink transport blocks that can be transmitted in one transmission time interval (TTI) results in the increase in the number of bits of an HARQ ACK/NACK signal for the downlink transport blocks. For example, if 8 downlink transport blocks are transmitted, it is necessary to transmit an 8-bit HARQ ACK/NACK signal.
In the 3GPP LTE, the PUCCH structure is designed on the basis of a 2-bit HARQ ACK/NACK signal. Therefore, it is required to design a PUCCH for carrying an HARQ ACK/NACK signal having the increased number of bits.
In addition, a great number of transport blocks are not always transmitted. Therefore, if the control channel is designed according to maximum capacity, it may be ineffective to transmit an ACK/NACK signal having a small bit size.
There is a need to design the control channel to have high reliability with respect to an ACK/NACK signal having a variable bit size.